Notes on Choosing a Fertilizer forum: Notes on Choosing a Fertilizer

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Feb 3, 2010 8:11 PM CST
Name: Al
5b-6a MI
A) Plant nutrients are dissolved in water
B) The lower the nutrient concentration, the easier it is for the plant to absorb water and the nutrients dissolved in the water - distilled water is easier for plants to absorb than tap water because there is nothing dissolved in distilled water
C) The higher the nutrient content, the more difficult it is for plants to absorb water and the nutrients dissolved in water
D) To maximize plant vitality, we should supply adequate amounts of all the essential nutrients w/o using concentrations so high that they impede water (and nutrient uptake). All that is in the "Fertilizer Thread" I posted a while back.

Let's imagine that we are going to fertilize our plants and examine how 2 different fertilizers might perform. We nearly always formulate a fertilizer program based on N needs because plants use more N than any other element. For the sake of this illustration, let's say that the plant needs 12 parts per gallon of N in the nutrient solution to prevent N deficiency, and that 50 parts per gallon of all the solids dissolved in the nutrient solution is the point at which water absorption becomes difficult & causes some degree of drought stress.

First, let's look at what happens when we supply a 3:1:2 ratio fertilizer - let's say we use 12-4-8 to supply the 12 parts of N needed. Because the P and K goes with the N, 12 + 4 + 8 = 24 parts of fertilizer applied. We know there are substantial other dissolved solids (many are nutrients, but some are unusable elements) in the solution besides the NPK, so let's set that amount at 12 parts. You can see that we now have the 24 + 12 parts or 36 parts of total dissolved solids in the solution, and we're still well under the 50 parts where water uptake becomes difficult.

Now lets shift gears to a 1:1:1 ratio fertilizer that has 12 parts of N. Obviously, it has 12 parts of P and 12 parts of K, too, so we're applying 36 parts of fertilizer to satisfy the same plant's need for 12 parts of N. If you add together the 12 parts of other dissolved solids like we did in the 3:1:2 illustration above, we are applying a solution of 48 parts per gallon to satisfy the need for 12 parts of N. What gives here?!

Looking deeper into this particular illustration, we can see that if we make a minor mistake and apply too much, or there are even a small amount of residual dissolved solids in the container solution, we are going to go easily beyond the safe 50 parts per gallon. On the other hand, if we reduce the amount of fertilizer applied so we're not dangerously close to the stress causing 50 parts per gallon, we fall below the 12 parts of N needed and have created a N deficiency.

The devil in the details tells us that the key to this second illustration lies in the fact that we unnecessarily applied much more P and K than the plant could use by using the 1:1:1 ratio 12-12-12.

This was simply an illustration to help you understand that even though plants have a tendency to take what they need and leave the rest, 'the rest' CAN have a profound effect on plant vitality/nutrition simply by being there, even though unused.

While using a 1:1:1 fertilizer as your 'all-purpose' choice is certainly possible, people have been doing it for years, using a fertilizer that is very close to what the individual plant is known to use (and an extremely high % of plants use closer to a 3:1:2 ratio than any other readily available blend) greatly widens the margin for application error by allowing you to apply at lower rates, and greatly increases the range between deficiency and toxicity levels of many of the elements in whatever fertilizer you choose.

Consider that perhaps instead of arbitrarily choosing a fertilizer without due consideration and using it as YOUR 'all-purpose' and 'the one I always reach for' blend, we might just be better off if we look for a fertilizer already labeled 'All-Purpose'. The odds are 3:1:2 that it will better suit your needs.

Jan 28, 2011 8:45 PM CST
Name: (Kevin51)
SouthEastern US Zone 7
I'm not getting old, I'm 'maturing'
Which would you say is the best fert. to use on a Mitrostigma and a Murray? thanks, Lee.
Jun 10, 2011 4:57 PM CST
Name: (Kevin51)
SouthEastern US Zone 7
I'm not getting old, I'm 'maturing'
What would you think better for my orchids: urea free or not? I grow most inside but have 4 outside.
Thank you

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